Kepler publishes first two planetary laws
Kepler publishes first two planetary laws
Overview
In 1609 Johannes Kepler published Astronomia Nova, distilled from Tycho Brahe's precise observations of Mars. Its first two laws — that planets move on ellipses with the Sun at one focus, and that they sweep out equal areas in equal times — replaced two millennia of perfect-circle astronomy with dynamical regularities a physical theory could explain. A third law followed in 1619, and together the laws became the empirical foundation on which Newton built universal gravitation.